Leveling device



Aug. 30, 1938. P. s. STEVENS LEVELING DEVICE Filed Oct. 16, 1936 3 Sheets-Sheet 1 fall/ fifma INVENTOR. 1 Wvf vA TTORNEYS. v

Aug. 30, 1938. p, 5, STEVENS 2,128,273

LEVELING DEVICE 3 Sheets-Sheet 2 Filed Oct. 16, 1936 #y: 1 sy ww,

ATTORNEYS.

Aug. 30, 1938.

P. S. STEVENS LEVELING DEVICE Filed Oct. 16, 1936 3 Sheets-Sheet 3 I N VEN TOR.

BY wWMW W ATTORNEYS.

Patented Aug. 30, 1938 UNITED STATES LEVELING DEVICE Paul S. Stevens,

South Milwaukee, Wis., assignor to Bucyrus-Erie Company,

South Milwaukee,

Wis., a corporation of Delaware Application October 16, 1936, Serial No.

corners of the base; each set supporting its corner of the base by means of an electrical, mechanical, or hydraulic jack. It is desired to control the automatic action of these jacks, by means of a level-responsive controlling device.

But leveling is not the only problem involved n this connection. Due to unevenness of the ground, it has been found necessary to automatically equalize the pressure on the four jacks. Also it has been found necessary to prevent any jack from overrunning predetermined extreme in or out positions. Optional manual control for each jack has also been found advisable.

Accordingly it is the object of my invention to provide a system of four jacks, which shall level automatically and equalize automatically, which shall optionally be controllable by the operator, and in which no jack can overrun.

A further object of my invention is to improve the details of each of these functions.

In addition to my principal object, above stated, I have worked out a number of novel and useful details, which will be readily evident as the description progresses.

My invention consists in the novel parts, and in the combinations and arrangements thereof, which are defined in the appended claims, and of which one embodiment is exemplified in the accompanying drawings, which are hereinafter particularly described and explained.

Throughout the description, the same reference number is applied to the same member'or to similar members.

Figure 1 is a perspective view, partly cut away, of the base of an excavator possessing a leveling system which includes the device of my in-.

vention. I

Figure 2 is a. vertical section of one of my jacks. Figure 3 is an enlarged elevation of my equalizing switch.

Figure 4 is an enlarged elevation of one of my limiting switches.

Figure 5 is an electric-wiring diagram of my complete invention.

Referring now to Figure 1, we see that H is the main base of the excavator or other machine, to which it is intended to apply my invention. This base is supported, by means of four hydraulic jacks I2, on four caterpillar tracpropelled by any unit I3 supports a piston so that when, relative .ted lines.

frame.

tion ground-engaging units l3, which may be convenient means, not shown.

Associated with each jack, there is a pump P, driven'by a motor M, and certain valves (not shown in Figure 1), all of which are operated and controlled through certain electric interlocks which will be hereinafter described.

Figure 1 is quite diagrammatic. to Figure 2, it will be seen that each traction M- through a universal joint l5. Piston l4 operates in cylinder It, the operating fluid being admitted to the space H. The piston I 4 is shouldered, as indicated at 8, to the cylinder Hi, the piston is forced downward far enough for the shoulder to abut element I9, the piston l4 and the cylinder 16 will be prevented from separating further. This extreme position of the piston with respect to the cylinder is shown by dot- Rigid with the top of the cylinder is an element 20, having a conv'exly curved upper bearing surface, which cooperateswith the concave under surface of a bearing element 2|, which is rigidly carried by a casting 22 attached to the main frame ll. Aligned through bearing elements 20 and 2| and casting 22 is a hole for the passage of bolt 23, which is inserted into the head of cylinder. The hole through these three elements is made somewhat larger than the bolt 23, so that there shall normally be a small clearance 24 at each side of the bolt. A similar small clearance 25 is shown between the lower portion of the cylinder I6 and the lower plate of the main The purpose of these clearances is to allow a slight lateral swing of the cylinder with respect to the main base Thus the cylinder is not rigidly connected to the base ll, its only connections being the bolt 23 and a brace (not shown) below the base, said brace being also capable of allowing a small motion of the cyl-- inder.

The upper end of bolt 23 is provided with a nut 26 and a lock nut 21. These are so adjusted that, when the curved bearing surfaces of elements 20 and 2| are in contact, there is a small clearance 28, of the order of one quarter inch, between the nut 26 and its seat on casting 22. The function of this clearance, which will be explained in detail later, is intimately connected with the equalizing switch 29, which is attached by a. bracket 30 to the under side of the lower plate of the main base ll. 1

Figure 3 shows. this equalizing switch considerably enlarged. Cylinder lfi carries a small Turning now unit I3, the clearance allows.

'Switch 42 is therefore a I derstanding of the lug 3I, upon which rides a wheel 32. The wheel is carried by the small arm ofa bell crank 33, whose pivot is carried by bracket 30. To the end of the long arm of bell crank 33 is pivoted a link 34, whose other end is pivoted to the crank 35. The different lengths of the two arms of bell'crank 33 cause a small motion of wheel 32 to effect a large motion of the crank 35. Rigid with this crank, but inside the box, is the moving arm which opens or closes the equalizing switch, according as lug 3I pushes wheel 32 upward or allows the wheel to move downward. The inner construction of this switch, being well known, is not shown. Normally, base II is resting on the cylinder, wheel 32 is in its extreme upward position, and the switch is open. If, owing to a slight sinking of traction spring (not shown) within the box of switch 29 compels the wheel 32 to follow lug 3|, and closes the switch.

Attached to a convenient part of the traction base I3, as at 36, Figure 2, is a wire rope 31 which passes through a hole 38 in the lower plate of main base II, to a pulley, 39 supported by the upper plate of the base I I, and thence to a switchactuating weight 40. This weight slides on a vertical guide 4|, and serves to actuate .limit switches 42, 43 and 44, which are placed as follows. Limit switch 42 is positioned so as to be actuated by the weight 40 when the piston isquite near, but not at, its all ou position; that is, when the particular corner of main base I I is almost as far above traction unit I3 as possible. Limit switch 43 is so positioned that it will be contacted by the weight 40 only when the piston is at its all ou position, and main base II is as far above traction unit' I3 as the mechanical construction Limit switch 44 is so positioned that it will be actuated only when the piston is at its all in position; that is, when the particular corner ofmain base II is as near the traction unit I3 as the mechanical construction will allow.

Switch 42 is shown enlarged in Figure 4. A wheel 45 is carried on an arm 45 pivoted at 41. In rigid relation to arm 45 but inside the box is the moving arm which closes the switch when wheel 45 is contacted by the sliding weight 40. I normally open switch.

Switches 43 and 44 are similar to switch 42, except that they are normally closed switches, and are open only when contacted by sliding weight 40. a

The inner mechanism of these switches is well known and therefore not shown. Each contains a spring, to restore it to normal position, when it is not contacted by the weight 40.

Turning now toFigure 1, I shall consider the lower left hand corner of the main base as corner I-, the remaining corners being numbered counterclockwise around the figue. For the proper unelectricalhook-up, as shown on Figure 5, it is essential to remember that corner 3 is diagonally opposite to corner I, and that corner 4 is diagonally opposite to corner 2.

At the right hand margin of Figure 5, I have placed opposite certain of the electrical instrumentalities the capital letters: A, B, C, D, E, F, G, H, J. Each of these letters refers collectively to the instrumentaiities that are to be found across the page on a level with the particular capital letter. All the instrumentalities opposite to any one letter are of the same kind.

The diagram naturally divides itself into four vertical bands, each associated with a motorwhen the main 28 closes, lug 3| sinks. A

' being held in fixed pump set, and the motors and pumps are numbered accordingly, the numbering corresponding to the numbering'of the corners, as stated above.

With the exception only of the E, G, and J elements, all the devices which are shown (Figure 5) in the vertical band which includes MI and PI, more particularly relate the corner I and AI is the limit switch 44 of corner I.

BI, the left hand one of the pair, is limit switch 43 of corner I.

BI, the right hand one of the pair, is limit switch 42 of corner I.

Cl is a solenoid which actuates a bleeder (not shown), to bleed cylinder I4 of corner I into a sump.

DI is a magnetic switch to actuate the motorpump system at corner I. t

E represents manual controls, located at the operators station, their use to be explained later I Fl is limit switch 29 of corner I.

The function of the G and H switches will be stated later.

The J elements are four mercury switches, arranged in pairs, the two elements of each pair relation to each other, though the pair may be tilted either way as a unit. These depart from the numbering system used above, in that, beginning switches must be numbered J I, J3; J2, J4.

The pair of switches, J I-J3, comprise a switch unit used to bring corners I and 3 to the same level. They might be merely regidly set on the main base in the diagonal between those corners, but in that case, because of the shortness of the individual elements, it would require a very considerable difference of level to cause them to function at all. Therefore I make use of the. device shown and described in the copending application of Luther E. Blanchett for improvements in leveling devices, filed May 13, 1936, Serial No. 79,594, which is owned by my assignee. This device, by multiplying the difference of level of the ends of a much longer base line, set in the line between corners I and 3, will tilt my mercury switch unit, J I -J 3, into operative position, for a difference of level of onlya few inches between corners I and 3.

The switch unit, 'J2-J4, is similarly arranged with respect to the diagonal from corner 2 to corner 4.

The single heavy line 5I represents the threephase power line to the motors. The control sys- 53, respectively at top and bottom of diagram.

A switch 54, when open, will entirely out out the automatic control, leaving the manual contem is supplied with power through wires 52 and at the left, the individual through wire 59 and the left hand one (43) of the BI switches, to wire 52; and closing magnet switch DI, thereby starting pump PI. At the same time, current will flow through the central switch point of HI, and wires 60, BI, 62, thereby actuating the solenoid C3, which will open a valve to bleed the cylinder at corner 3 into a sump. Neither the valve nor the sump is shown, they being of any convenient conventional form, and not constituting per se an inventive feature. These two actions will raise corner I and lower corner 3. When level is attained, mercury switch unit JIJ3 will again be in normal position, and consequently solenoid will be deenerg'ized, and HI, which is a spring switch, will open, and pumping and bleeding will stop.

Now let-us assume that the excavator is still not level, although PI has pumped the jack at comer I almost to its limit of travel. It is obvious that the excavator cannot be leveled further without lowering both corners 2 and 4. My invention automatically accomplishes this in the following manner.

Referring to Figure 2, we see that, in this case, as the cylinder I6 is forced upward with respect to the piston I4, the rising of the frame II at this corner will raise pulley 39; and, inasmuch as attachment 36 remains down, there will be a pull on rope 3!, which will raise weight40 with respect to frame II The weight 40 will first contact switch 42, which is the right hand one of the BI switches in Figure 5, thereby closing it.

The closing of the right hand BI switch (42) will energize C4 solenoid valve, thereby bleeding corner 4. As corner 3 is still bleeding, the side 3-4 of the excavator will be lowered. When corner 4 is sufiiciently below the level of opposite corner 2, the mercury switch unit J2J4 will be tilted into position to cause J4 to close the circuit through H4 and D4. This will set M4 and C2 into action, thereby pumping at corner 4 and bleeding'at corner 2. We are now both pumping and bleeding at corner 4, but the corner will remain at the same position, as the valves bleed at about the same rate as the pumps supply liquid.

When corner 2 reaches the level of corner 4, mercury switch unit J2'J4 will return to its normal position, opening H4 and discontinuing pumping by P4. Then leveling between corners I and 3 will continue as described above.

If corner I was originally a large amount below level, the complete cycle described above will be several times repeated.

If the above described action, resulting from the closing of switch 42, has not relieved the situation which closed this switch, namely the fact that the jack at the corner in question had been pumped almost to the limit of its travel, I provide the following safety device as a last resort. Further pumping of that jack results in further raising the weight 40 until it contacts switch 43, which is the lefthand one of the BI switches, thereby opening it. This will stop pumping by PI, but will not stop bleeding at the other three corners, and hence will not interfere with the effect of the closing of switch 42, already described.

The means for accomplishing these two successive remedies, called into play by the successive closing of switch 42 and opening of switch 43, will be described in certain of my claims as means associated with each corner, whereby maximum expansion of the jack at any corner will first lower the base by releasing the jack at 'one of the corners adjacent to the given corner, and

will then put an end to the expansion of the jack at the given corner.

In the foregoing discussion of the action of the BI switches of Figure 5) is closed. If corner 3 has bled to its limit, its piston is all in, and the sliding weight, 40 at corner 3, will open switch 44', which is switch A3 of Figure 5, and corresponds, at corner 3, to switch 44 at corner I, shown in Figure 2. This will prevent the bleeding of C4, and the consequent lowering of corner 4 in spite of the fact that switch BI right is still closed. The object of this is that, inasmuch ascorner I (which is low) has been raised as far as possible, and as corner 3 (which is high) has now been lowered as far as possible; no

to no good end.

This reaching the limit of possibility of further leveling, is not a defect of my invention; for obviously any leveling-device, which could be humanly designed, must of necessity have a limit beyond which it cannot function further.

It should be noted that this opening of switch 44' under the circumstances stated, then prevents merely such bleeding on the 2-4 diagonal as would normally be caused by lack of level 'of the I3 diagonal, and does not at all prevent bleeding on the 2-4 diagonal responsive to its own lack of level.

From all the foregoing discussion, it Will be seen that, in order to simplify my electrical hookup, I have provided a system which, in many cases, will operate circuitously through many cycles, before finally attaining a position of rest. The advantage of simplicity of construction, however, outweighs circuity of operation.

But, by the addition of an optional manual control, I can enable the operator to rapidly perform the first rough leveling operations by eye, after for example glancing at a spirit level; leaving it to, my automatic controls to finish up the job in aminimum of cycles.

To this end, I have provided an optional manual control entirely independent of the automatic con- 0 trol. The eight similar elements on the E line. Figure 5, represent push-buttons. Following from wire 53, through wires 63 and 64, it will be seen that if button 65 is pushed, MI will be actuated and PI will pump corner I. If button 66 is pushed, solenoid CI will bleed corner I. It is to be noted that wire 63 takes power, regardless whether main switch 54 is open or closed.

The push buttons will also be of use in many circumstances that readily suggest themselves. All the foregoing falls under the generic term: leveling. .I shall now discusswhat I call equalizing.

In case corner I is originally relatively high, and has been bled to its limit before corner 3 is pumped up to its limit, the continued pumping of P3 will tend to raise the main base oil the cylinder at corner 2 or corner 4, thus causing what I shall call in my claims the dangling of the jack at that corner. This dangling will cause the entire jack to drop as far as permitted by the small clearance 28 at the corner where the dangling occurs, thereby closing the corresponding equalizing switch23' (as shown at F2 and F4 in Figure 5). As F2 and F4 are in parallel,

. Then, as'soon as the clearance switch, 29, will open,

the closing of either of them will close theright hand G switch, 61. Current will therefore flow via wires 68, 69, to switch ii'l, where it divides. One line, via wires i and II will and the other, via wire 12,.will start pump P4. 28 is open again (provided the corresponding clearance paired with it has not, in the meantime, closed), the F and pumping at corners 2 and i will stop. During and after all this pumping the process of leveling between corners l and will automatically continue until completed.

The equalizing by means of switch 29 also comes automatically into play whenever, by reason of heavy digging over one corner, or for any other reason, a corner sinks an amount suflicient to'cause an undesirable teetering of the excavator, but not enough to actuate the corresponding mercury switch. My discussion has, for simplicity, cated upon the assumption that corner i was initially too low, and corner 3 initially too high; thus calling for the expansion of the jack at corner i, and the contraction of the jack at corner 3. As the four verticalbands of Figure plainly with respect tothe four corners ofthe excavator,

been prediit is not considered necessary to discuss the conventive.

- made,

sequences of any other unlevel condition.

Qwing to the capillary repulsion between glass and mercury, the mercury switches will not close the circuits of which they constitute parts until there is a three or four inches betweenjtwo corners. This is of advantage as it is not desirable that the mercury units be affected by ordinary vibrations. But the mercury connection having Once been thecohesion of the mercury will maintain it until the difference of level is nearly zero.

Main switch 54, Figure 5, is arranged to be automatically opened when the machine is travelling, as it is not necessary to correct for every unevenness of the ground over which the machine is momentarily passing. The precise means of this automatic opening is not claimed as in- Any mechanic or electrical engineer could readily devise means to this end, such, for example as solenoid switch 54 shown in Figure 5. The solenoid of this switch is actuated by being in some electric circuit 13 which is energized during propelling.

In Figure l, I have shown at 18, a pipe connecting the pipes which supply liquid from two adjacent pumps to their respective cylinders. This pipe 58 has a valve 49, conventionally shown, which is automatically opened when the machine is travelling. This allows the four caterpillar units to accommodate themselves freely to unevennesses of the ground, with the load approxi-' mately equally distributed among them.

Although I have described my invention as used with hydraulic jacks, it is obvious that. it can be equally well applied to a system using electrical or mechanical jacks.

And although I have described my invention with respect to four independent ground-engaging units, the term ground-engaging unit is to.be understood as also comprising one end of a ground-engaging device, which device supports the base through a jack at each end of such device.

When herein I refer to equalizing the pressure on the four jac I realize that I am not using this phrase in its literal physics sense.- Rather I mean so pressing start pump P2;

exhibit the symmetry of the circuits end to the expansion of difference of level of at leastthe jacks against the ground, with the main base level, that each jack will bear its normal share of the load, as the center of gravity and digging reactions shift from point to point during the digging cycle.

Having now described and illustrated one form of my invention, I wish it to be understood that my invention is not to be limited to the specific form or arrangement of parts hereinbefore described, except in so far as'such limitations are specified in the appendedclaims.

I claim:

1. In combination: a base;

,ners of the base will expand the jack at the lower of these two corners, and will release the jack at the higher of the two corners; and, with respect to each corner, means associated with that corner, whereby the jack at that corner will first lower the base by releasing the jack at one of the corners adjacent to the given corner, and then will put an the jack at the given corner.

2. In combination: a base; gaging units; four jacks, one connected to and supported by, each ground-engaging unit, and connected to and supporting the base; means to expand each jack; means to release each jack, so as to permit for theexpanding and releasing of the jacks; two level-responsive switch devices, each responsive to the diflerence in level of a pair of opposite corners of the base; each switch device being so coupled with the lectric controls that a difierence in level of a pair of opposite corners of the base will expand the jack at the lower of these two comers, and will release the jack at the higher of the two corners; and, with respect to each corner, means associated with that corner, whereby maximum expansion of the jack at that corner will lower the base by releasing the jack' at one of the corners adjacent to the given corner, and will put an end to the expansion of the jack at the given corner.

3. In combination: .a baseyfour ground-engaging units; supported by connected to and supporting expand each jack; means to release each jack, so

as to permit it to contract; electric controls for the expanding and releasing of the jacks; two'- level-responsive switch devices, each responsive to the difference in level of a pair of opposite corners of the base; each switch device being so coupled with the electric controls that a difference in level of a pair of oppositecorners oi the base will expand the jack at the lower ofthese two corners, and will release higher of the two corners, with respect to each corner, means associated with that corner, whereby maximum expansion of the jack at that corner will lower the base by releasing the jack at one of the corners adjacent to the given corner, and will put an end to the expansion of the jack at the given corner; and means whereby four ground-en it to contract; electric controls the jack at the- I fourground-engaging units; four jacks, one connected to and devices, each responsive maximum expansion of maximum contraction of the jack at the corner opposite to the given corner will render inoperative the first-mentioned effect of the last-mentioned means.

4. In combination: a base; four ground-engaging units; four jacks, one connected to and supported by each ground-engaging unit, and connected to and supporting the base; means to expand each jack; means to release each jack, so as to permit it to contract; electric controls for the expanding and releasing of the jacks; two

level-responsive switch devices, each responsive to the difierence in level of a pair of opposite corners of the base; each switch device being so coupled with the electric controls that a difference in level of a'pair of opposite corners of the base will expand the jack at the lower of these two corners, and will release the jack at the higher of the two corners; and means whereby the dangling of any jack, without contributing to the support of the base, will cause the expansion of that jack.

5. In combination: a base; four ground-engaging units; four jacks, one connected to and supported by each ground-engaging unit, and connected to and supporting the base; means to expand each jack; means to release each jack, so as to permit it to contract; electric controlsfor the expanding and releasing of the jacks; two level-responsive switch devices, each responsive to the difference in level of a pair of opposite corners of the base; each switch device being so coupled with the electric controls that a difierence in level of a pair of opposite corners of the base will expand the jack at the lower of these two corners, and will release the jack at the higher of the two corners, with respect to each corner, means associated with that corner, whereby maximum expansion of the jack at that corner will lower the base by releasing the jack at one of the corners adjacent to the given corner, and will put an end to the expansion of the jack at the given corner; and means whereby the dangling of any jack, without contributing to the support of the base, will cause the expansion of that jack. 1

6. In combination: a base; four ground-engaging units; four jacks, one connected to and supported by each ground-engaging unit, and connected to and supporting the base; means to expand each jack; means to release each jack, so as to permit it to contract; electric controls for the expanding and releasing of the jacks; two level-responsive switch devices, each responsive to the difference in level of a pair of opposite corners of the base; each switch device being 80 coupled with the electric controls that a difference in level of a pair of opposite corners of the base wll expand the jack at the lower of these two corners, and will release the jack at the higher of the two corners, with respect to each corner, means associated with that corner, whereby maximum expansion of the jack at that corner will lower the base by releasing the jack at one of the corners adjacent to the given corner, and will put an end to the expansion of the jack at the given corner; means whereby maximum contraction of the jack at the corner 0pposite to the given corner will render inoperative the first-mentioned effect of the last-mentioned means; and means whereby the dangling of any Jack, without contributing to the support of the base, will cause the expansion of that jack.

7. In combination: a base; four ground-ena in units; four jacks, one connected to and supported by each ground-engaging unit, and connected to and supporting the base; means to expand each jack; means to release each jack, so as to permit it to contract; electric controls for I the expanding and releasing of the jacks; two

level-responsive switch devices, "each responsive to the difference in level of a pair of opposite corners of the base; each switch device being so coupled with'the electric controls that a difierence in level of a pair of opposite corners of the base will expand the jack at the lower of these two corners, and will release the jack at the higher of the two corners; and means automaticaliy rendering all the foregoing means inoperative during propelling.

8. In combination: a base; four ground-engaging units; four jacks, one connected to and supported by each ground-engaging unit, and connected to and supporting the base; means to expand each jack; means to release each jack, so as topermit it to contract; electric controls for the expanding and releasing of the jacks; two level-responsive switch devices, each responsive to the difference in level of a pair of opposite corners of the base; each switch device being so coupled with the electric controls that a difference in level of a pair of opposite corners of the base will expand the jack at the lower of these two corners, and will release the jack at the higher of the two corners, with respect to each corner, means associated with that corner, whereby maximum expansion of the jack at that corner will lower the base by releasing the jack at one of the corners adjacent to the given corner, and will put an end to the expansion of the jack at the given corner; means whereby the dangling of any jack, without contributing to the support of the base, will cause the expansion of that jack; and means automatically rendering all the foregoing means inoperative during propelling.

9. In combination: a base; four ground-engaging units; four jacks, one connected to and supported by each ground-engaging unit, and connected to and supporting the base; means to expand each jack; means to release each jack, so as to permit it to contract; electric controls for the expanding and releasing of the jacks;

two level-responsive switch devices, each respon-' sive to the difference in level of a pair of opposite corners of the base; each switch device'being so coupled with the electric controls that a diiference in level of a pair of opposite corners of the base will expand the jack at the lower of these two corners, and will release the jack at the higher of the two corners, with respect to each corner, means associated with that corner, whereby maximum expansion of the jack at that corner will lower the base by releasing the jack at one of the corners adjacent to the given corner, and will put an end to the expansion of the jack at the given corner; means whereby maximum contraction of the jack at the corner opposite to the given corner will render inoperative the firstmentioned effect of the last-mentioned means; means whereby the dangling of any jack, without contributing to the support of the base, will cause the expansion of that jack, and meansautomatically rendering all thevforegoing means inoperative during propelling.

10. In combination: a base; four ground-engaging units; four jacks, one connected to and supported by each ground-engaging unit, and connected to and supporting the base; means to expand each jack; means to release each jack, so as to permit it to contract; electric controls S smears for the expanding and releasing of the jacks; two electric controls for the expanding and releasing. level-responsive switch devices, each responsive to of the jacks; two level-responsive switch devicefi, the difference in level or a pair of opposite corners each responsive to a departure from level of one of the base; each switch device being so coupled of a pair of normally horizontal lines in the bas with the electric controls that a difference in level at substantially right angles to each other; means of a pair of opposite corners of the base will exwhereby said controls automatically eil'ect levelpand the jack at the lower or these two corners, ing of the base; and means whereby said controls and will release the jack at the higher of the automatically eflect equalizing'of the pressure on two corners, with respect to each corner, means thesupporting units.

associated with that corner, whereby maximum 12. In combination: a base; four ground-enll expansion of the jack at that corner will lower gaging units; tour jacks, one connected to and the base by releasing the jack at one of the supported by each ground-engaging unit, and

' corners adjacent to the given corner, and will connected to and supporting the base; means to put an end to the expansionvot the jack at the expand each jack; means to release each jack,

given corner; means whereby the dangling of so as to permit it to. contract; and, with respect 1 any Jack, without contributing to the support 01 to each ground-engaging unit, a control mechthe base, will cause the expansion of that lack, anism, connected to two elements which tend to and means for optional manual control of the move relative to each other whenever and only expansion and releasing of each jack. when that unit dangles, this control mechanism 11. In combination: a base; a plurality of being operable by that relative motion to posiground-engaging units; a plurality of jacks, one tively actuate the expanding means of the jack connected to and supported by each ground-enat that corner. so that the Jack will constitute gaging unit, and connected to and supporting a positive support for the base at that comer.-

the base; means to expand each jack; means to release each jack, so as to permit it to contract; PAUL s. s'ravrms. s 

